1. Field of the Invention
The present invention relates to the art of printing, and more particularly to apparatuses and methods for printing and verifying precision of bar codes, and correcting subsequent prints of bar codes on labels and sheets.
2. Description of the Prior Art
Bar codes in various forms are now familiar as identifying and pricing indicia for various commodities and material, and are used to record pricing information and material handling and locating information on commodities and products in applications as varied as grocery store checkouts and inventory control. Bar codes typically comprise vertical bars of dark, non-reflective lines of varying widths interspaced by highly light reflective spaces of varying widths. A group of such bars and spaces, representing a "symbol" is scanned by optical scanners reflecting light off the bars and spaces to photodetectors which distinguish the difference between the dark bars and the reflective spaces, and of their relative widths.
To distinguish between the relative widths of bars and of spaces, processing of the information from the scanned code must distinguish the time of the scanner's pass over of the bars and spaces Usually, great tolerances must be afforded to the different, varying widths in order to accommodate a substantial variability in the speed or velocity of the optical scanner, and to accurately distinguish between bars, or non-reflective areas and spaces, or highly reflective areas.
It is greatly desired to reduce the widths of the bars and spaces generally conceded to tolerances without sacrificing accuracy in the optical scanning, in order to increase the information in a given space and to increase the reliability of the information read-out in the scanning procedure. For example, it has been shown to print an alignment mark to trigger the commencement of an optical bar code scanning at a precise point, with the objective of reducing the symbol space, as is shown in U.S. Pat. No. 4,349,741 to Bobart et al., where the alignment mark is scanned contemporaneously with the printing so as to position the scanner at the center of the bar code, and thus increase the accuracy in the scanning of the code.
Thermal printers have been known and preferred in many applications where non-impact printing is a desired goal. Thermal printers are characterized by the controlled application of precise amounts of heat, either by a print element head or pixel to a paper sensitive to the temperature to produce an optically readable print, or to a thermally sensitive ribbon adjacent the paper to be printed, in which by the application of heat to the ribbon, optically readable material is melted onto the paper. In such thermally sensitive printing, precision in the temperature of the printing elements is important. Methods and apparatuses for sensing the temperature and correcting or controlling the temperature to the print head in response to the sensed temperature have been described, for example, in the dot matrix printer controls of U.S. Pat. Nos. 3,577,137 to Brennan, Jr. and 4,449,033 to McClure, et al.
While greatly improving the quality of thermal printing, it is still desired to have an apparatus which self-corrects the precise widths of the bars and spaces in a bar code during the printing process in response to a contemporaneous read-out of the immediately preceding printed code, and potentially independent of temperature sensing. It is further desired to have a bar code printer having contemporaneous read-out and verification of the printed code, and automatic rejection of code which does not fit within pre-defined width dimension tolerances of precision for the bars and spaces. It is a further desired objective to have bar code printers automatically positioning the edge of labels to be printed in such a printing and verifying printer. It is desired yet further to have a printer with the self-correcting and contemporaneous verification features in a non-impact printer.